This research is centered around rodent models of protective immune responses to the lymphatic filarial nematode, Brugia pahangi. We compared the infectivity in gerbils of two B. pahangi strains, one passed successfully 22 times previously in gerbils and a second derived directly from dogs. There was no significant difference in 200-day recoveries, sex ratios, sizes or locations of adult worms. Microfilaremias were significantly higher in the gerbil strain animals, in nine of eleven blood sample collections. Lewis rats showed lower susceptibility to infection. Most animals given 100 infective larvae developed a microfilaremia but patency was sometimes greatly delayed and values were often quite erratic or low. Even large numbers of larvae (400), cannot establish peritoneal infections. Preliminary results suggested that there is no differential sex susceptibility in terms of microfilaremia. A major difficulty was our inability to locate or recover adult worms in long-term infections. The nude (congenitally athymic) mouse (3CH/HeN nu/nu) is quite susceptible to infection. Average worm recoveries were 14.5 percent of inoculum. About 3/4 of adult worms localized in the heart/lungs. Mean microfilaremia climbed rapidly in both sexes of nudes. It appeared to plateau around four months, with maximum values around 600 mf/20mm of blood. Normal mice (C3H/HeN nu/plus or plus/plus were quite resistant to infection and few yielded worms after 35 days post-inoculation. These and other results suggested that resistance to B. phangi was thymus-dependent and can be restored to nudes through reconsitution of nude mice to study the role of various immune components in the protective responses against filarial larvae.